1. Field of the Invention
This invention in general relates to vibration damping apparatus, and more particularly to such apparatus of the active type.
2. Description of the Prior Art
The vibration behavior of most rigid structures is characterized by several modes of vibrational motion each associated with a particular resonant frequency. Methods utilized to reduce the vibration include the use of a passive damping material such as a viscoelastic compound or other energy absorber in contact with the structure, to introduce damping. Under many circumstances, however, the damping material must cover an appreciable fraction of the surface to be damped and for large structures the material not only significantly increases the total weight due to the heavy damping material, but uses volume which is often limited, and seldom achieves a damping ratio of more than 10%.
To overcome the deficiencies of a passive damping arrangement, use is made of an active vibration isolation system which includes, in general, a sensor which senses the movement of the vibrating body and translates this movement into an electrical signal. This signal is provided to a driver unit which counteracts the vibrational movement. Prior art systems of this type include a piezoelectric sensor, or other type of displacement sensor, interposed between a vibrating body and a base and which produces a signal proportional to the displacement of the body relative to the base. The signal is provided to an active driver connected to the body to counteract the movement and in essence the arrangement simulates a spring isolation.
The present invention utilizes a sensor-driver arrangement, however one wherein a dashpot or shock absorber is simulated, even on a body in free space, so as to absorb the vibrational energy and reduce the velocity of vibration of the body.